The present invention relates to a protecting circuit which can control an output current, against short-circuit of an output terminal of an AC adapter.
In conventional AC adapters for charge, an output voltage thereof is temporarily zero, when a large current flows or an output terminal of an AC adapter is short-circuited. Thereafter, the conventional AC adapters for charge can be charged again by flowing a load current of approximately 500 mA. To prevent heating due to the charge, means for preventing the heating is provided for a casing of the AC adapters.
However, as mentioned above, the output voltage is temporarily zero and the load current of approximately 500 mA thereafter flows to charge the AC adapter again, when the output terminal of the AC adapter is short-circuited and, then, there is a danger in that the output terminal of the AC adapter is repeatedly short-circuited. In this case, since the means for preventing the heating has limitation, there is a danger in that the load is damaged.
Accordingly, it is an object of the present invention to provide a preventing circuit against short-circuit of an output terminal of an AC adapter, which needs no means for preventing the heating by controlling an output current.
The present invention is devised by paying attention to a fact that a value of a controlled constant-voltage is reduced to make the above means for preventing the heating unnecessary, only when it is within an available range.
A protecting circuit in the present invention comprises a current output switching transistor, a first switching transistor, a second switching transistor, a switch control amplifier, and a constant-current control amplifier.
A power source is connected to an emitter of the current output switching transistor, and an output terminal is connected to a collector of the current output switching transistor. The first switching transistor is turned on when a value of a controlled constant-voltage is approximately a value when a voltage difference between a voltage of the power source and an output voltage is over an available range. The second switching transistor is connected to be turned on when the first switching transistor is turned on. The switch control amplifier which controls a switching operation so that the current output switching transistor is turned off when a voltage of the power source is a predetermined voltage or less. The constant-current control amplifier controls a constant current when the second switching transistor is turned on and enters a protecting operation state.